An atmospheric measurement station (MARS) was established in cooperation with the University of Mérida, Physics Department in the vicinity of the city Mérida, Venezuela (8°N, 71°W). This station takes advantages of the high altitude site Pico Espejo (4765 m) for measurements with a DOAS instrument and a microwave sensor. Pico Espejo is accessible with the world's highest cable car. The DOAS instrument at this station was operated from March 2004 to November 2008 when it had to be removed due to interruption of cable car services. As a result of extended periods of power outage, only a partial time series is available.
The Pico Espejo station is characterized by its high altitude, facilitating stratospheric measurements in a tropical region that is unperturbed by tropospheric pollution and the large tropospheric water vapour burden usually found at these latitudes. Therefore it is an ideal site for both stratospheric research and the validation of satellite instruments such as GOME, SCIAMACHY and MIPAS. Using the off-axis measurements of the DOAS instrument it is also possible to investigate trace gases in the free troposphere, and to quantify the effects of local pollution from Mérida and additionally long range transport from the fires in Southern America.
The DOAS instrument in Mérida was located inside the MARS station building. It comprised a temperature stabilized grating spectrometer equipped with a cooled CCD detector. The instrument was connected to a telescope, which was located outside the building, with a quartz fibre bundle and a number of electrical and control connections. The telescope had two viewing ports: One in zenith direction, the other one towards the horizon (off-axis). In the off-axis direction, up to 4 angles could be observed sequentially in the range of -5° to 25° elevation by means of a motorized mirror. Both line-lamp and white light calibration lamp were integrated in the telescope box and are used for daily calibration measurements. More on the instrument and measurement principle can be found on our MAXDOAS page.
The instrument was fully automated and controlled remotely through an internet connection to Bremen. To ensure continuous operation during power black outs, it was connected to an UPS shared with the microwave instrument.
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